Simulations of physical systems are widely available online, with no cost, and are ready to be used in our classrooms.1,2 Such simulations offer an accessible tool that can be used for a range of interactive learning activities. The Jovian Moons Applet2 allows the user to track the position of Jupiter's four Galilean moons with a variety of viewing options. For this activity, data are obtained from the orbital period and orbital radii charts. Earlier experiments have used telescopes to capture the orbital motion of the Galilean moons,3 although observation of astronomical events and the measurement of quantities may be difficult to achieve due to a combination of cost, training, and observing conditions. The applet allows a suitable set of data to be generated and data analysis that verifies Kepler's third law of planetary motion, which leads to a calculated value for the mass of Jupiter.

2.
Jovian Moons
Applet
,
Jürgen
Giesen
, www.jgiesen.de/JovianMoons/.
Other reference sites include The NASA J-SAT website. Refer to “WebSights: Kepler's third law activity using the NASA J-SAT website to collect data,”
Phys. Teach.
44
,
191
(
March 2006
).
3.
Roger B.
Culver
, “
An experiment for studying Kepler's laws and planetary masses from satellite motions
,”
Am. J. Phys.
39
,
1404
(
Nov. 1971
), and Alfred Sutton, “The mass of Jupiter,” Phys. Teach. 11, 488–489 (Nov. 1973).
4.
NASA Solar System Exploration, Jupiter Moons page, solarsystem.nasa.gov/planets/.
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